Particle Diffusion Coefficient Research Articles

Probing Diffusion of Single Nanoparticles at Water–Oil Interfaces

The diffusion of nanoparticles at a water-alkane interface is studied using fluorescence correlation spectroscopy. Hydrophilic and hydrophobic quantum dots of 5, 8, and 11 nm radius are used. A slow-down of nanoparticle diffusion at the liquid-liquid interface is observed. The effect is most evident when the viscosities of both liquid phases are similar, here, at the water-decane interface. In this case, the interfacial diffusion coefficients of the hydrophilic particles are 1.5 times and those of the hydrophobic particles 2 times lower than the corresponding bulk values.

Effective diffusion and effective drag coefficient of a brownian particle in a periodic potential

We study the stochastic motion of a Brownian particle driven by a constant force over a static periodic potential. We show that both the effective diffusion and the effective drag coefficient are mathematically well-defined and we derive analytic expressions for these two quantities. We then investigate the asymptotic behaviors of the effective diffusion and the effective drag coefficient, respectively, for small driving force and for large driving force. In the case of small driving force, the effective diffusion is reduced from its Brownian value by a factor that increases exponentially with the amplitude of the potential. The effective drag coefficient is increased by approximately the same factor. As a result, the Einstein relation between the diffusion coefficient and the drag coefficient is approximately valid when the driving force is small. For moderately large driving force, both the effective diffusion and the effective drag coefficient are increased from their Brownian values, and the Einstein relation breaks down. In the limit of very large driving force, both the effective diffusion and the effective drag coefficient converge to their Brownian values and the Einstein relation is once again valid.

Interaction of albumin and γ-globulin molecules with gold nanoparticles in water solutions

The molecular mobility of scattering particles in water solutions of albumin and γ-globulin proteins with gold nanoparticles was investigated by the dynamic light scattering method. The dependences of translation diffusion coefficients of particles at various concentrations of the solution components have been obtained. It was revealed that in a wide range of concentrations of the solution components there is no interaction of gold nanoparticles with albumin, whereas gold nanoparticles form complexes with γ-globulin molecules.

Overview of FTU results

New FTU ohmic discharges with a liquid lithium limiter at IP = 0.7–0.75 MA, BT = 7 T and ne0 ⩾ 5 × 1020 m−3 confirm the spontaneous transition to an enhanced confinement regime, 1.3–1.4 times ITER-97-L, when the density peaking factor is above a threshold value of 1.7–1.8. The improved confinement derives from a reduction of electron thermal conductivity (χe) as density increases, while ion thermal conductivity (χi) remains close to neoclassical values. Linear microstability reveals the importance of lithium in triggering a turbulent inward flux for electrons and deuterium by changing the growth rates and phase of the ion-driven turbulence, while lithium flux is always directed outwards. A particle diffusion coefficient, D ∼ 0.07 m2 s−1, and an inward pinch velocity, V ∼ 0.27 m s−1, in qualitative agreement with Bohm–gyro-Bohm predictions are inferred in pellet fuelled lithized discharges. Radio frequency heated plasmas benefit from cleaner plasmas with edge optimized conditions. Lower hybrid waves penetration and current drive effects are clearly demonstrated at and above ITER densities thanks to a good control of edge parameters obtained by plasma operations with the external poloidal limiter, lithized walls and pellet fuelling. The electron cyclotron (EC) heating system is extensively exploited in FTU for contributing to ITER-relevant issues such as MHD control: sawtooth crash is actively controlled and density limit disruptions are avoided by central and off-axis deposition of 0.3 MW of EC power at 140 GHz. Fourier analysis shows that the density drop and the temperature rise, stimulated by modulated EC power in low collisionality plasmas are synchronous, implying that the heating method is the common cause of both the electron heating and the density drop. Perpendicularly injected electron cyclotron resonance heating is demonstrated to be more efficient than the obliquely injected one, reducing the minimum electric field required at breakdown by a factor of 3. Theoretical activity further develops the model to interpret high-frequency fishbones on FTU and other experiments as well as to characterize beta-induced Alfvén eigenmodes induced by magnetic islands in ohmic discharges. The theoretical framework of the general fishbone-like dispersion relation is used for implementing an extended version of the HMGC hybrid MHD gyrokinetic code. The upgraded version of HMGC will be able to handle fully compressible non-linear gyrokinetic equations and 3D MHD.

Measurements of pollen grain dispersal in still air and stationary, near homogeneous, isotropic turbulence

The dispersal of ragweed, pine and corn pollen as well as polystyrene spheres in still air and stationary, near homogeneous, isotropic turbulence (HIT) was investigated using high-speed, digital inline holographic cinematography enabling Lagrangian tracking of the particles. Mean still air settling velocities were similar as reported literature values. Small discrepancies were most likely related to species/size differences and water content of the grains. Near-HIT was generated by loudspeakers mounted on the corners of a 40 cm 3 chamber and the turbulent flow field at the center of the chamber was validated using stereoscopic Particle Image Velocimetry (PIV). Results showed near homogeneity and near isotropy with mean velocities 5–10 times smaller than the corresponding rms values of velocity fluctuations. The turbulent kinetic energy dissipation rate was determined from the PIV data sets and used to calculate the Kolmogorov scales and Taylor microscales. Experiments were carried out for two different loudspeaker amplifications corresponding to Taylor microscale Reynolds numbers, R λ =144 and 162, respectively. The mean settling velocity in turbulent conditions was in all cases higher than the corresponding still air value, the difference becoming smaller as particle Stokes numbers increased. For the present conditions, the still air particle settling velocity was lower than the rms values of air fluctuating velocities. As a result, dispersion was dominated by inertia and for a given R λ , particle fluctuating velocity autocorrelations fell more rapidly as the particle Stokes number decreased; corresponding particle diffusion coefficients also decreased. Transverse particle diffusion coefficients were lower than those in the direction of gravity in agreement with the continuity effect. Under the present range of experimental parameters, results showed that inertial particles (0.6<St<11) in highly turbulent conditions disperse more effectively than the air.

Deposition of charged aerosol nanoparticles in diffusion batteries

The deposition of weakly charged aerosol nanoparticles onto fibers in a diffusion battery designed to measure the diffusion coefficients of particles is considered. The fiber collection efficiancies as functions of particle size and charge are determined by the numerical solution of the equation of convective diffusion in a system of parallel uncharged fibers located normal to a flow. It is shown of effect of the single charge of nanoparticles produced by a differential mobility analyzer on their deposition is negligible and may be ignored when calibrating diffusion batteries.

Characterization of the poloidal asymmetries in the ISTTOK edge plasma

Edge plasma in/out and up/down asymmetries have been directly investigated on ISTTOK with a probe system that simultaneously samples the plasma at four poloidal angles. An experimental investigation of the asymmetries is presented as a function of the direction of plasma current and toroidal magnetic field. The edge plasma parameters show significant in/out asymmetries in both equilibrium and fluctuating parameters. Asymmetries favoring the outboard side are observed in density, turbulent particle flux and diffusion coefficient independently of the toroidal magnetic field and plasma current directions, suggesting a ballooning-like transport. This interpretation is supported by the characteristics of the fluctuations as they are found to be more intermittent on the outboard side. In addition, smaller up/down asymmetries reversing with the reversal of the toroidal magnetic field direction and favoring the ion ∇B drift direction are found. Significant poloidal asymmetries were also measured in the parallel flow that can be attributed mainly to Pfirsch–Schluter flows.

Diffusion of Particles on a Fluctuating Surface

Using kinetic Monte Carlo simulations, we have investigated within the framework of a simple lattice–gas diffusion model, the diffusion of particles adsorbed onto a lattice with moving surface atoms. Dynamic surface reconstruction was found to substantially increase the particle diffusion coefficient and change the activation energy.

Magnetophoresis, sedimentation, and diffusion of particles in concentrated magnetic fluids

A dynamic mass transfer equation for describing magnetophoresis, sedimentation, and gradient diffusion of colloidal particles in concentrated magnetic fluids has been derived. This equation takes into account steric, magnetodipole, and hydrodynamic interparticle interactions. Steric interactions have been investigated using the Carnahan-Starling approximation for a hard-sphere system. In order to study the effective interparticle attraction, the free energy of the dipolar hard-sphere system is represented as a virial expansion with accuracy to the terms quadratic in particle concentration. The virial expansion gives an interpolation formula that fits well the results of computer simulation in a wide range of particle concentrations and interparticle interaction energies. The diffusion coefficient of colloidal particles is written with regard to steric, magnetodipole and hydrodynamic interactions. We thereby laid the foundation for the formulation of boundary-value problems and for calculation of concentration and magnetic fields in the devices (for example, magnetic fluid seals and acceleration sensors), which use a concentrated magnetic fluid as a working fluid. The Monte-Carlo methods and the analytical approach are employed to study the magnetic fluid stratification generated by the gravitational field in a cylinder of finite height. The coefficient of concentration stratification of the magnetic fluid is calculated in relation to the average concentration of particles and the dipolar coupling constant. It is shown that the effective particle attraction causes a many-fold increase in the concentration inhomogeneity of the fluid if the average volume fraction of particles does not exceed 30%. At high volume concentrations steric interactions play a crucial role.

Secondary Ion Mass Spectrometry Study of Thermal Diffusion of Au Nanoparticles in Porous SiO&lt;SUB&gt;2&lt;/SUB&gt; Matrices

Migration of Au nanoparticles by thermal diffusion into porous SiO2 matrix substrates has been studied using secondary ion mass spectroscopy (SIMS). When the samples having four different porosities were annealed at T = 410 K for 1.5 h, no noticeable variations in the thermal diffusion of Au nanoparticles were observed. All the measured diffusion coefficients of Au particles, were an order of 10(-15) cm2/s at 300-410 K in a very limited interfacial region. Regardless of their porosities, the pores must be discontinuous, which acts as a diffusion barrier to block the continuous diffusion of Au particles.

Diffusion of Particles Adsorbed on a Dynamically Disordered Reconstructive Surface

We have considered the diffusion of particles in a dynamically disordered medium in the framework of a simple lattice-gas model of a reconstructive surface. Using kinetic Monte Carlo simulations we have investigated the diffusion of particles over the host lattice with moving atoms. The dynamic lattice reconstruction increases substantially the particle diffusion coefficient and changes its activation energy.

Applicability of the Taylor-Green-Kubo formula in particle diffusion theory

Diffusion coefficients of particles can be defined as time integrals over velocity correlation functions, or as mean square displacements divided by time. In the present paper it is demonstrated that these two definitions are not equivalent. An exact relation between mean square displacements and velocity correlations is derived. As an example of the applicability of these results so-called drift coefficients of energetic particles are discussed. It is explained why different previous approaches in drift theory provided contradicting results.

Development of a semiclassical method to compute mobility and diffusion coefficient of a Brownian particle in a nonequilibrium environment

We explore the issue of a quantum-noise-induced directed transport of an overdamped Brownian particle that is allowed to move in a spatially periodic potential. The established system-reservoir model has been employed here to study the quantum-noise-induced transport of a Brownian particle in a periodic potential, where the reservoir is being modulated externally by a Gaussian-colored noise. The mobility of the Brownian particle in the linear response regime has been calculated. Then, using Einstein's relation, the analytical expression for the diffusion rate is evaluated for any arbitrary periodic potential for the high-temperature quantum regime.

Communication: Turnover behavior of effective mobility in a tube with periodic entropy potential.

Using Brownian dynamics simulations, we study the effective mobility and diffusion coefficient of a point particle in a tube formed from identical compartments of varying diameter, as functions of the driving force applied along the tube axis. Our primary focus is on how the driving force dependences of these transport coefficients are modified by the changes in the compartment shape. In addition to monotonically increasing or decreasing behavior of the effective mobility in periodic entropy potentials reported earlier, we now show that the effective mobility can even be nonmonotonic in the driving force.

The role of exopolysaccharide produced by Lactococcus lactis subsp. cremoris in structure formation and recovery of acid milk gels

The effect of exopolysaccharide (EPS) produced by Lactococcus lactis subsp. cremoris JFR1 on milk fermentation and recovery of gel structure after stirring was studied using rheology and diffusing wave spectroscopy. Skim milk fermented with an EPS-non producing (EPS −) strain of L. lactis served as control. The production of EPS was not sufficient to affect the stages preceding aggregation. No differences were found in the changes in diffusion coefficient, radius and turbidity parameter of casein particles up to the point of gelation between EPS + and EPS − samples. However, the presence of EPS significantly increased the storage modulus and viscosity of milk gel, and reduced the ability of the gel to recover after shearing. This study indicates that although the present EPS does not affect the preliminary stages of gelation, it is produced in sufficient quantities after gelation to significantly alter the structure of milk gels in their set or stirred form.

Anomalous nanoparticle diffusion in polymer solutions and melts: A mode-coupling theory study

Mode-coupling theory is employed to study diffusion of nanoparticles in polymer melts and solutions. Theoretical results are directly compared with molecular dynamics simulation data for a similar model. The theory correctly reproduces the effects of the nanoparticle size, mass, particle-polymer interaction strength, and polymer chain length on the nanoparticle diffusion coefficient. In accord with earlier experimental, simulation, and theoretical work, it is found that when the polymer radius of gyration exceeds the nanoparticle radius, the Stokes-Einstein relation underestimates the particle diffusion coefficient by as much as an order of magnitude. Within the mode-coupling theory framework, a microscopic interpretation of this phenomenon is given, whereby the total diffusion coefficient is decomposed into microscopic and hydrodynamic contributions, with the former dominant in the small particle limit, and the latter dominant in the large particle limit. This interpretation is in agreement with previous mode-coupling theory studies of anomalous diffusion of solutes in simple dense fluids.

Experimental study and simulation of vibrated fluidized bed drying

The paper addresses numerical simulation for the case of convective drying of seeds (fine-grained materials) in a vibrated fluidized bed, analyzing agreement between the numerical results and the results of corresponding experimental investigation. In the simulation model of unsteady simultaneous one-dimensional heat and mass transfer between gas phase and dried material during drying process it is assumed that the gas–solid interface is at thermodynamic equilibrium, while the drying rate (evaporated moisture flux) of the specific product is calculated by applying the concept of a “drying coefficient”. Mixing of the particles in the case of vibrated fluidized bed is taken into account by means of the diffusion term in the differential equations, using an effective particle diffusion coefficient. Model validation was done on the basis of the experimental data obtained with narrow fraction of poppy seeds characterized by mean equivalent particle diameter ( d S,d = 0.75 mm), re-wetted with required (calculated) amount of water up to the initial moisture content ( X 0 = 0.54) for all experiments. Comparison of the drying kinetics, both experimental and numerical, has shown that higher gas (drying agent) temperatures, as well as velocities (flow-rates), induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity. Bed temperature differences along the bed height, being significant inside the packed bed, are almost negligible in the vibrated fluidized bed, for the same drying conditions, due to mixing of particles. Residence time is shorter in the case of a vibrated fluidized bed drying compared to a packed bed drying.

Solution NMR Spectroscopy and Protein Interaction Studies of Membrane Proteins in Nanodiscs

Insolubility of integral membrane proteins (IMPs) in buffer often prevents their investigation with biophysical methods developed for soluble proteins. First, solution NMR of large proteins or complexes is limited by slow rotational diffusion. IMPs in cellular membranes or liposomes are beyond the size limit of solution NMR. Therefore, detergent micelles or bicelles are often used for solubilization and NMR studies of IMPs. Unfortunately, detergents may destabilize the native structure or compromise the activity of proteins. Nanodiscs are a lipid-based, detergent free, relatively small and very promising new model membrane system. We inserted a recombinantly produced and 15N, 13C-labeled fragment of human CD4 into nanodiscs. This polypeptide comprises the membrane-spanning and the cytoplasmic domains of CD4. Our NMR data demonstrate the feasibility of solution NMR spectroscopy on IMPs in nanodiscs. Second, surface plasmon resonance (SPR) is predominantly utilized in interaction studies of soluble proteins. Incorporation of IMPs into nanodiscs provides a close to native environment to the membrane protein and results in a water-soluble proteolipid particle that might be amenable to standard SPR-based methodology. We reconstituted a decahistidine-tagged IMP into nanodiscs and studied binding between the nanodisc-inserted IMP and a PentaHis monoclonal antibody (mAb) immobilized on the surface of a CM5-sensorchip. For comparison, we also determined the affinity of the decahistidine-tagged soluble domain of the same IMP toward the immobilized PentaHis mAb. Binding affinities were almost identical in both cases. However, the association and dissociation rate constants were found to differ, which is in agreement with the distinct diffusion coefficients of the soluble analyte particles. Our data indicate that nanodisc-inserted IMPs can serve as analyte in interaction studies of membrane proteins.

Molecular dynamics simulation of corrosive particle diffusion in benzimidazole inhibitor films

Diffusion of corrosive particles inside inhibitor films consisting of 2-mercaptobenzimidazole (2-SH-BI), 2-aminobenzimidazole (2-NH 2-BI), 2-methylbenzimidazole (2-CH 3-BI), and benzimidazole (BI) was investigated by molecular dynamics simulation. Diffusion coefficients of various corrosive particles in the films were calculated, following the same order of 2-SH-BI < 2-NH 2-BI < 2-CH 3-BI < BI. Fractional free volume, interaction between corrosive particles and films, and mobility of films were also investigated to illustrate the microscopic diffusion mechanism. As a result, it can be inferred that the order of inhibition efficiency is 2-SH-BI > 2-NH 2-BI > 2-CH 3-BI > BI, which is in good accordance with published experimental results.

Diffusion of particles over dynamically disordered lattice

Using kinetic Monte Carlo simulations we have investigated the diffusion of particles over a lattice with moving atoms in the framework of a simple lattice-gas model of a reconstructive surface. The particle migration over the static and dynamic lattices differs considerably. The dynamic lattice reconstruction changes substantially the particle diffusion coefficient. The Arrhenius dependencies are quantitatively different. An interesting peculiarity of the particle migration is the formation of defectons--local lattice deformations around the adsorbed particles. In certain ranges of the system parameters (jump rates of the substrate atoms and adsorbed particles) the adsorbed particles cause local displacements of the nearest substrate atoms, resulting in deeper adsorption sites and lower jump rates. Such particle self-trapping produces a characteristic minimum on the Arrhenius dependencies.